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A popular sport throughout 2013 will involve pundits parsing every movement of the bond and stock markets and change in fund flows, looking for the first definitive sign of a broader investor shift out of bonds and into stocks.

As media outlets and Twitter users are apt to trip over one another to be the first to weigh in on any news event or to call any market bottom or top, so will we be awash in attempts to declare the onset of such a shift, what Bank of America last year termed a coming "great rotation" out of fixed income and into equities.

THE IDEA IS THAT YIELDS on all sorts of bonds will soon fall so low that they will no longer compensate for the risks of inflation or rising interest rates. Great herds of investors will then begin migrating back into stocks, even if equities still fail to inspire, because they will be less uninspiring than bonds. Many have called for such a shift to commence late this year, pending continued improvements in economic trends, while bearing in mind that the Federal Reserve is still doing its best to suppress rates.

But after early January brought an equity rally and a Treasury selloff, some observers said that the shift had begun. The Wall Street Journal ran a story to that effect last week, citing weekly data showing investors adding money to U.S. stock funds for the first time in six months.

Still, fixed-income funds have sustained overall inflows, and indeed it looks too early to write off bonds. The foremost reason for now is the clash over raising the debt ceiling that looms next month. The buildup to the fiscal-cliff mess kept a lid on Treasury rates in late 2012, and many expect a similar dynamic in the coming weeks.

During the last debt-ceiling showdown, in the summer of 2011, Standard & Poor's stripped the U.S. of its triple-A credit rating, citing concern that the federal government was more willing to incur a self-inflicted wound than to achieve fiscal accord.

Paradoxically, the immediate result of the rating cut was a Treasury-market rally. Investors viewed the downgrade as more of an indicator of broader domestic economic ills than a sign that the U.S. was at greater actual risk of defaulting on its debt.

Fast-forward to last week, when Fitch Ratings indicated that it, too, will likely cut the U.S. sovereign rating if Washington can't reach a more meaningful agreement this time around. The 10-year Treasury yield ended the week at 1.843%, versus 1.866% the Friday before.

"If we have another one-minute-to-midnight deal, which we think is damaging to confidence and the recovery, and simply sets up another deadline for six months later, it's not something which from our perspective would be consistent with retaining the triple-A," David Riley, head of Fitch's sovereign-rating team, told The Wall Street Journal last week. Moody's Investor's Service, the third major rating agency, also has hinted at a U.S. sovereign-rating downgrade in 2013.

AS IN 2011, DON'T EXPECT the debt-ceiling fight to stoke optimism about the American economy. That should keep a cap on rates for now.

And don't expect any subsequent downgrade to push Treasury yields much higher, either. Anthony Valeri, a strategist at LPL Financial, recently looked back at 11 sovereign-rating downgrades over the past two decades and found reliably muted bond-market reactions, with 10-year government yields rising by just 0.01%, on average, a week after each downgrade, and 0.07% three months later.

"The average yield change has been negligible following downgrades," Valeri writes. "Other factors, such as economic growth or central-bank actions, have been focal drivers of interest rates for highly rated countries."